The present invention relates to near-azeotropic compositions utilizable as refrigerating fluids in circuits working according to the Rankine cycle. More particularly the present invention relates to compositions having zero ODP (Ozone depleting Potential) value and low GWP (Global Warming Potential) and VOC (Volatile Organic Compound) values, utilizable as low environmental impact substituents of dichlorodifluoromethane (R-12).
R-12 has been widely used as refrigerating fluid for circuits working at middle-high evaporation temperatures, typical of the domestic refrigeration and of the housing and motorvehicles air-conditioning. At present the manufacturing and marketing of R-12, and generally of chlorofluorocarbons, are submitted to restrictive rules in all the world for applications of this kind because of the alleged destroying power of this product on the ozone layer present in the stratosphere.
Therefore, the need of finding other products or compositions capable of effectively replacing R-12 without causing environmental damages, is particularly felt. For this purpose, it was suggested the use of hydrofluorocarbons (HFC) or hydrochlorofluorocarbons (HCFC), whose depleting potential with respect to ozone (ODP) is very low or even zero, as very low is also their contribution to the so called xe2x80x9cgreenhouse effectxe2x80x9d, measured by the GWP.
Among the various substitute products of R-12 up to now proposed, the most known is 1,1,1,2-tetrafluoroethane (R-134a), a product having a low environmental impact (ODP=0); GWP=0.35) and properties similar to those of R-12. However R-134a shows as refrigerant a coefficient of performance lower than that of R-12 and, on the other side, it is not compatible with the conventional lubricants of mineral type, therefore its use as refrigerating fluid requires on the one hand a new planning of the refrigerating circuit to avoid higher energy consumptions and on the other hand the substitution of the lubricating oil with another compatible oil. The oil conventionally used with R-12 is indeed of the mineral type while that required by R-134a belongs to the esterified polyols class and the two lubricants are not compatible.
In order to overcome the drawbacks shown by R-134a, alike other single refrigerants, the use of mixtures containing HFC has been proposed. However, if mixtures are used, other inconveniences are encountered. First of all, because of the different volatility of the components, fractionation occurs when passing from liquid phase to vapour phase and viceversa, with a remarkable variation in the condensation and evaporation temperatures, so as to impair even considerably the efficiency of the refrigerating circuit. Moreover, the filling up of the refrigerant, necessary in consequence of unavoidable losses from the refrigerating plant, cannot be carried out with the original mixture, but it is necessary to proportion the various components according to the exact composition of the mixture remained after fractionation, so as to restore the initial optimum composition. Lastly, if the mixture contains a more volatile, inflammable component, the vapour phase enriches in such component until the inflammability point is reached, with evident hazard during its use. Similarly, if the inflammable component is less volatile, it concentrates in the liquid phase, giving rise to an inflammable liquid.
In order to avoid such drawbacks, it is therefore convenient to use mixtures having an azeotropic behaviour, i.e. mixtures characterized in that they behave as pure fluids. However, the obtainment of azeotropic mixtures is an extremely rare event, since it requires a particular combination of boiling temperatures and deviations from the ideal behaviour of the various components. Therefore, the study of refrigerating mixtures has been directed to the obtainment of xe2x80x9cnear-azeotropicxe2x80x9d mixtures. The definition, among those suggested until now, which better suits the purposes of the present invention, is that according to which a near-azeotropic behaviour occurs if the percentage pressure variation in consequence of a 50% evaporation of the liquid (indicated as xcex94p/p per 100) at 25xc2x0 C. is lower than 15% (in this respect see the article by D. A. Didion and D. B. Bivens in Int. J. Refrig., vol. 13, p. 163 and following, 1990).
A further characteristic desirable for the substituents of chlorofluorocarbons (CFC)-based refrigerants, as already mentioned, is that they shall not virtually require any modifications of elements, materials and, generally, components of the system in which they operate: in this case we can speak of xe2x80x9cdrop-inxe2x80x9d substituents. In particular, it would be advantageous having a product or a mixture soluble in the lubricating mineral oils commonly used with conventional refrigerants, or soluble in an oil compatible with the oils used at present. In such a way, before introducing the new refrigerant, complex operations of complete discharging, accurate washing and drying of the refrigerating plants would be avoided.
In EP 299614 various near-azeotropic mixtures of halocarbons are proposed as substituents of R-12 in the refrigerating field. The mixtures of this kind on the one hand still show not zero ODP values because of the presence of chlorine atoms in one or more components, on the other hand, according to what ascertained by the Applicant, require the use of an alkylbenzenic lubricating oil, with the drawbacks described above.
In EP 565265 are described mixtures containing R-134a, an hydrocarbon selected from propane, propylene or isobutane and optionally octafluoropropane (R-218). Although these mixtures are an improvement compared with R134a used alone, they show the drawback of a notable deviation from the azeotropic behaviour. See in particular col. 3, lines 13-15.
In EP 638623, in the name of the Applicant, mixtures as substituents of R-12 and R-502 are described, however some of them have not the feature of being xe2x80x9cdrop-inxe2x80x9d, while for others it has been found that they do not have sufficient chemical stability during the use.
The Applicant has unexpectedly found that HFC-based mixtures containing hydrocarbons as hereinunder defined, have near-azeotropic behaviour, are non-flammable up to an hydrocarbon content of about 4% by weight or only slightly flammable for an hydrocabon content higher than 4% and up to 10% by weight and are characterized by vapour pressure curves such as to make them particularly suitable as substituents for refrigerants R-12, by enjoying moreover of the feature of being xe2x80x9cdrop-inxe2x80x9d. Such mixtures are moreover characterized by a very low or zero environmental impact, expressed in terms of ODP, GWP and VOC.
Therefore, object of the present invention are ternary mixtures, utilizable as refrigerating fluids, essentially of the following types:
said mixtures having the feature that the percent variation of the vapour pressure after the 50% evaporation of the liquid at the temperature of 25xc2x0 C. is comprised between 0.5 and 15% of the vapour pressure before said evaporation and preferably between 0.5 and 7%. The n-butane is usually a commercial product which can contain up to 10% of isobutane. Similarly, isobutane is usually a commercial product which can contain up to 10% of n-butane.
Preferably A, B and C mixtures contain 75-86% of R134a, 4-20% of R-125 and 2-4% of hydrocarbon (R-600 and/or R-600a); the D, E and F mixtures contain 75-93% of R-134a, 5-20% of R-227ea and 2-4% of hydrocarbon (R-600 and/or R-600a), since these mixtures result non flammable. Unexpectedly, as already said, the mixtures containing only a little more than 4% of hydrocarbon result slightly flammable. Moreover the mixtures containing n-butane are preferred to those containing isobutane; this was unexpected as the azeotropic or near-azeotropic behaviour usually is found more easily when the boiling points of the components are closer.
Generally, the mixtures object of the present invention, being constituted by more refrigerants, show the advantage of a greater flexibility and therefore they meet better than one single component the thermodynamic and thermophysical characteristics required for a certain refrigerating circuit configuration.
As already said, the above mentioned mixtures are substituents of R-12 of drop-in type, as they can be used in the existing equipments concerning refrigeration at middle evaporation temperature, in particular in the domestic refrigeration, without needing the replacement of mechanical parts or of conventional mineral lubricating oils. This feature was unexpected if it is considered that the drop-in substituents of R-12 previously known contain chlorine. Once dissolved in the lubricating oil the invention mixtures, unlike those previously used, do not cause, also at high temperatures and for long contact times, noticeable alterations in the chemical-physical characteristics of both the oil and the metal surfaces usually present in the refrigerating circuits, revealing therefore a good chemical stability.
Moreover, thanks to the near-azeotropic characteristics, depending on the content of hydrocarbons, said mixtures show no or at most a slight tendency to fractionate into inflammable liquids or vapours also after a substantial evaporation of around 50% by weight.
It has also been unexpectedly found that even small percent amounts of n-butane or isobutane present in these mixtures allow to noticeably improve the solubility of conventional mineral lubricants in HFC, notoriously incompatible with these oils. Moreover is has been noticed that also when the solubility of the oil in the refrigerant is not optimum, the lubricating oil is unexpectedly capable of coming back from the evaporator, which represents the critical zone of the circuit, to the compressor, thus maintaining lubrication and avoiding wear phenomena of the compressor mechanical parts and undesired oil accumulations in the exchangers.